Field of the Invention
The present invention relates to a method for preparing a lithium iron phosphate nanopowder coated with carbon.
Description of the Related Art
As technical development and consumption of mobile devices increase, the demand for secondary batteries as energy sources is suddenly increasing. Among such secondary batteries, lithium secondary batteries having high energy density and voltage, a long life cycle, and a low self-discharge rate are being commercialized and widely used.
Common lithium secondary batteries use lithium cobalt oxide (LiCoO2) as the main component of a cathode active material. However, since the cobalt oxide containing lithium is unstable and expensive, the mass production of lithium secondary batteries including thereof is difficult.
Recently, lithium iron phosphate (LiFePO4) compound having a voltage against lithium of −3.5 V, a high volume density of 3.6 g/cm3, and a theoretical capacity of 170 mAh/g, as well as good stability at high temperature, and being cheap when compared to the lithium cobalt oxide, is being viewed as a suitable cathode active material for a lithium secondary battery.
As methods for preparing the lithium iron phosphate compound, a solid-state reaction method or a liquid-state reaction method such as a hydrothermal synthesis method and a supercritical hydrothermal synthesis is known. Recently, a glycothermal synthesis method is using a non-aqueous solvent such as ethylene glycol or diethylene glycol as a reaction solvent has been developed. According to the hydrothermal synthesis method and the supercritical hydrothermal synthesis method, the preparation of the lithium iron phosphate nanopowder is performed under high temperature and high pressure conditions, giving rise to safety concerns. In addition, according to the glycothermal synthesis method, the control of the particle size and the particle size distribution of the lithium iron phosphate nanopowder may be difficult.
The lithium iron phosphate has a relatively lower electric conductivity when compared to other cathode active materials including lithium. Particularly, lithium cobalt oxide (LiCoO2) has the electric conductivity of about 10−4 S/cm and lithium manganese oxide (LiMn2O4) has about 10−5 S/cm. On the contrary, the electric conductivity of lithium iron phosphate is 10−9 S/cm, which is smaller by about 10,000 times or more. Therefore, cathode material particle are necessary to be prepared as small nano-sized particle to supplement the low electric conductivity of the lithium iron phosphate material, and a conductive layer having good electric conductivity is necessary to be formed on the surface of the cathode material particle to increase the electric conductivity.